Surface energy balance characteristics of a heterogeneous urban ballast facet

Abstract

Surface energy balance measurements were conducted during a study period from June to September 2002 at a goods station in Osnabrück, Germany. The inner-urban facet with a surface area of 0.5 km2 was predominantly covered by ballast. Turbulent latent and sensible heat fluxes were estimated by a modified Bowen ratio method; the ground heat flux was calculated from temperature gradients and a laboratory derived thermal conductivity. The thermal behaviour of the ballast facet in comparison to other urban surfaces is somewhat 'two-sided'. Due to its thermal properties it behaves similarly to urban surfaces during the day, with high surface temperatures and large near-surface temperature amplitudes. About one-third of the incoming energy is converted into the turbulent sensible heat flux and 20 % is converted into the ground heat flux. The latent heat flux at the site is of minor importance (16 %). During the night, however, the thermal behaviour is more comparable to a 'rural-like' surface than to an urban facet. The ballast surface cools significantly and the sensible heat flux is directed towards the surface. Due to the heterogeneity of the study site the energy balance shows a closure gap of around 30%. Reasons for the energy balance non-closure are discussed. © Inter-Research 2005.